This invention relates to a novel misch metal-nickel type multi-component alloy useful for the occlusion of hydrogen, which alloy is capable of occluding a large amount of hydrogen in the form of hydride and, upon slight heating, readily and quickly releasing the occluded hydrogen.
Since hydrogen in an inexhaustible, clean substance easy to transport and store, it is attracting increasing attention as a new energy source to take the place of fossil fuels.
Hydrogen has long been stored either in a gaseous form or in a liquefied form. Since the development of a method capable of storing hydrogen in the form of hydride as occluded in a metal, various substances useful for such occlusive storage of hydrogen have been perfected.
The conditions which a hydrogen-occluding substance is required to meet are that the substance is inexpensive and abundant in supply, that it is easily activated and has a high capacity for the occlusion of hydrogen, that it possesses a proper dissociation equilibrium pressure near room temperature, that it only requires a low hydrogen pressure to permit its conversion to hydride, that it occludes and releases hydrogen reversibly at a high speed.
The transition metals such as Ti, Zr, La, and Mg have been known to form hydrides. These metals in the form of hydrides exhibit extremely high thermal stability; they do not release hydrogen until their temperature is elevated to high levels exceeding 300.degree. C. As substances for the occlusion of hydrogen, therefore, these metals have very poor practicability. In recent years, various alloys such as Ti-Ni, Ti-Co, Ti-Fe, La-Ni, Mg-Ni, Mm (misch metal)-Ni, and Mm-Co alloys have been developed. They invariably have numerous disadvantages which stand in the way of their actual adoption as substances for the occlusion of hydrogen. Of the various alloys enumerated above, those including Ti, La, and Mg are thermally as stable as the aforementioned single metals of Ti, La, and Mg or they occlude and release hydrogen at a very slow speed and do not permit easy activation. The metals which go to make up these alloys are required to possess very high levels of purity and, accordingly, pose a difficult economic problem. With these alloys, since their capacities for the occlusion of hydrogen are delicately affected by the purity of hydrogen to be occluded, the hydrogen given to be occluded is inevitably limited by the requirement that it must possess a high purity.
In the Ti-Fe alloy, for example, substantially no occlusion of hydrogen occurs under application of a hydrogen pressure of 50 kg/cm.sup.2 at room temperature, nor is it possible activate the alloy under these conditions. For this alloy to effect occlusion of hydrogen, its temperature must be elevated to a high level in the range of from 400.degree. C. to 500.degree. C. and the activation of the alloy requires the operation of occlusion to be repeated several times.
Comparison of the Mm-Ni alloy and the Mm-Co alloy reveals that the former has a high capacity for occlusion of hydrogen but involves a high dissociation equilibrium pressure and the latter has a low dissociation equilibrium pressure but suffers from a small capacity for occlusion of hydrogen. Further, the Mm-Ni alloy has a disadvantage that its activation requires a high hydrogen pressure of the order of 80 to 90 kg/cm.sup.2, consumes much time or necessitates a number of activation treatments and, at the same time, the occlusion and release of hydrogen takes much time.
Many inventions have been perfected to provide improved alloys such as Mm.sub.1-x Ca.sub.x Ni.sub.5 alloy (U.S. Pat. No. 4,096,639), MmNi.sub.5-x Co.sub.x alloy (U.S. Pat. No. 4,147,536), MmNi.sub.5-x A.sub.x alloy (U.S. Pat. No. 4,222,770), MmNi.sub.5-x Cr.sub.x-y A.sub.y alloy (U.S. Ser. No. 192,809, dated Oct. 1, 1980), and Mm.sub.1-x Ca.sub.x Ni.sub.5-y A.sub.y alloy (U.S. Ser. No. 222,351, dated Jan. 5, 1981). These alloys are noted to possess gradually improved properties as hydrogen-occlusion alloys. The Mm.sub.1-x Ca.sub.x Ni.sub.5 alloy enjoys a low cost of production but suffers from a high dissociation equilibrium pressure. The MmNi.sub.5-x Co.sub.x alloy is characterized by approximating the Mm-Co alloy in dissociation equilibrium pressure and the Mm-Ni alloy in capacity for the occlusion of hydrogen. The MmNi.sub.5-x A.sub.x alloy is improved in the properties such as the speed of hydrogen occlusion, the activation and the dissociation equilibrium pressure which are important for hydrogen occlusion alloys. The MmNi.sub.5-x Cr.sub.x-y Ay alloy exhibits a constant dissociation pressure over a wide range of hydrogen/metal atomic ratio, i.e. it posesses a small flatness factor. The Mm.sub.1-x Ca.sub.x Ni.sub.5-y A.sub.y alloy is characterized by its low cost. With these alloys, however, the pressures for activation and those for hydrogen occlusion are invariably on the order of 50 kg/cm.sup.2. For practical purposes, all these alloys are required to be further improved in their respective properties. Particularly, their pressures for activation and their pressures for hydrogen occlusion are desired to be lowered. No matter how excellent the property of a given alloy may be with respect to the occlusion and release of hydrogen, the alloy has absolutely no practicability when it requires some tens of atmospheres of hydrogen pressure for the activation of the alloy itself and for the occlusion of hydrogen. The container for enclosing the alloy itself, therefore, cannot be given a very simple construction.
An object of this invention is to provide an alloy for the occlusion of hydrogen, which combines all the properties necessary for the occlusion of hydrogen and, at the same time, permits the activation of the alloy and the occlusion of hydrogen to be effectively performed at low hydrogen pressures.